1. Field of the Invention.
The present invention relates to cordless telephone systems and to methods of controlling switching operations in such systems, and is useful for telephone systems employing TDMA (time division multiple access) and also for hybrid systems employing TDMA in conjunction with other access and modulation methods, e.g. to those employing both TDMA and CDMA-TDD (code-division multiple access-time division duplex).
The invention is particularly, but not exclusively, useful in systems employed for interfacing mobile telephony traffic to CATV (cable television ) plant.
2. Description of the Related Art
At the present time, cordless telephone systems are being developed which make use of signal exchanging components that perform both broadcast transmission and receiving functions, e.g. RADs (remote antenna drivers) and MEXs (microcell extenders), for exchanging radio telephone signals with mobile cordless handsets. The signal exchanging components are connected by signal conduits or transmission lines, e.g co-axial cables or optical cables, to basestations, which in turn are connected to the public switched telephone network, or some similar switched network.
The signal exchanging components are spaced apart from one another and may be distributed so that their coverage zones overlap one another to form a shaped antenna pattern, or distributed array antenna pattern. The coverage zones thus form so-called roamer corridors, along which the handset user can travel without the need for call hand-off processing for maintaining a communications link.
However, the applicant has now found that when such cordless telephone systems employ certain types of modulation, e.g. broadband TDMA, problems arise as the user moves through the areas of overlap of the coverage zones of adjacent ones of the signal exchanging components.
As discussed in greater detail below, these problems arise because of timing differences between the signals exchanged by the handset with two or more of the signal exchanging components. As a consequence of the timing offsets resulting from the different signal path lengths between the basestation and a first one of the signal exchanging components, on one hand, and between the basestation and a second one of the signal exchanging components, on the other hand, the handset may be unable to demodulate the composite signal when the time offset is significant compared to the bit period of the signals.
It is also known to provide a basestation with a pair of antennas which are spaced apart from one another, e.g. so that one of the antennas may avoid interference from surrounding objects which may adversely affect the operation of the other antenna.
With this arrangement, the basestation can switch from one to the other of these antennas, depending upon the strength of the signals received from the handset or some other measure of signal quality. This switching between the antennas is controlled in the basestation by one of a number of known diversity algorithms.
The present inventor has now found that the known antenna diversity switching algorithms or similar algorithms can be adopted to control the transmission and reception functions in the signal exchanging components, in a distributed array such as that described above, so as to overcome the above-discussed signal timing problem.